The invention relates to a process for the granulation of sludge—and optionally dust-form material, in which the material to be granulated is mixed with hydratable or hydraulically settable solids and water, which solids react exothermically with water and act as binder in the mixture, where the solids are allowed to hydrate or set fully or partly, and where the mixture containing the fully or partly hydrated or set solids is shaped into granules. The invention furthermore relates to an apparatus for carrying out the process.
The granulation of sludges with hydraulically setting solids is widely known from the prior art.
Thus, for example, EP 0 031 894 A2 discloses mixing water-containing sludge with a hydratable or hydraulically settable solid and granulating the mixture. In this process, burnt lime and sludge are conveyed to a mixing and granulation device by means of two conveying screws which in each case initially convey burnt lime and sludge separately, the mixing and granulation device being formed by the two conveying screws, which now operate in one another.
In this way, a transfer point for burnt lime and sludge is created which is sufficiently far removed from the point of metering of the burnt lime and therefore avoids problems resulting from the exothermic reaction of burnt lime with the sludge water.
In EP 0 805 786 B1, sludges initially produced as dusts in the reduction of iron ore with a reduction gas and deposited in a scrubber in the form of sludges are mixed with burnt lime and, if desired, coal dust and subsequently granulated.
A common feature of the two processes mentioned is that the binder properties arising for the solids on mixing the moist sludges with the hydratable or hydraulically settable solids are dependent on the moisture content of the sludges. Since, however, the moisture content of sludges is generally not constant, this results in varying process conditions during admixing of the solids and consequently in a non-constant quality of the granules. Simultaneous granulation of dusts is—without additional addition of water—only possible to the extent to which the sludges contain an excess of water.